The production of microelectronic devices, particularly for applications in the field of radiofrequency or power, requires the placing of components on a support substrate having a high electrical resistivity and good thermal conductivity.
Indeed, a high resistivity makes it possible to limit high-frequency interactions between transistors (the field line penetration in the substrate causing parasitic effects).
A good thermal conductivity is necessary to discharge the heat generated by high-frequency or high-power device operation.
According to a known solution, these devices may be produced on SOI (an acronym of the term “Silicon On Insulator”) type substrates, wherein the silicon support substrate (or a part thereof) is highly resistive.
In this way, document US 2009/321873 describes a structure successively comprising a silicon support substrate, a layer of high-resistivity silicon, a layer of silicon oxide and a thin layer of silicon wherein the components are formed.
Document US 2007/032040 describes an SOI substrate comprising a silicon support substrate having an electrical resistivity greater than 3000 Ohm·cm, a layer of silicon oxide and a thin layer of silicon wherein the components are formed.
However, these substrates involve the drawback of having a low thermal conductivity, particularly due to the presence of a relatively thick layer of silicon oxide (SiO2), which is a poor heat conductor.
The thermal conductivity of such SOI substrates, since the oxide thickness exceeds about 50 nm, may then be limited by the conductivity of this silicon oxide, of the order of 1 to 2 W/mK, which is insufficient for the intended applications.
According to a second known solution, the components may be produced on a first substrate, for example, a silicon substrate, and after the production thereof, the components may be transferred onto a final support substrate made of sapphire, which is a material having an electrical resistivity of the order of 1014 Ohm·cm.
Such an approach is presented, for example, in document U.S. Pat. No. 6,944,375.
However, sapphire has a thermal conductivity of 30 to 40 W/mK, which is considered to have scope for improvement for the intended applications.
An oxide layer is inserted between the layer supporting the components and the sapphire substrate to facilitate bonding.
However, as explained above, this oxide layer may form a thermal barrier preventing heat dissipation within the sapphire substrate.
Moreover, sapphire substrates are relatively costly, particularly for diameters larger than 150 mm.
One aim of the present invention is thus of providing a support substrate for a device for radiofrequency or power applications.
More specifically, this support substrate may have both a high electrical resistivity, i.e., greater than 3000 Ohm·cm, and a thermal conductivity at least equally as good as that of silicon (preferably greater than 30 W/mK), while being less costly than sapphire.
This substrate should be suitable for being manufactured to form large wafers, i.e., typically having a diameter greater than 150 mm.
This support substrate should also be suitable for the manufacturing process of the device and, particularly, have the required thermal properties (in particular, in terms of thermal expansion coefficient and temperature resistance) according to the process defined.